W2WDX
| Joined: | Fri, Sep 3rd 2010, 03:26 | Roles: | N/A | Moderates: | N/A |
Latest Posts
| Topic | Author | Posted On |
|---|---|---|
| Is "balanced" (as in baluns, etc.) a myth? | ag7ov | on 10/9/18 |
| I am a retired concert sound engineer, so I know a thing or two about audio. That being said, an amplifier to speaker connection is unbalanced. The input of that amplifier can be either balanced or unbalanced, however in the industry we tend to used balanced for low-level signals, principally to maintain higher s/n ratios. The levels on loudspeaker outputs are so high-level this is of little concern since the signal is significantly way above noise. Additionally, a loudspeaker motor device is an unbalanced device. Balanced, unbalanced. This is basic electronic theory. Whether audio frequencies or RF, when carried in electronic circuits all are electromagnetic waves with the same fundamental characteristics. They are all AC in nature. A balanced signal is one where there is positive and negative, referenced to ground. The positive and negative are usually at equal and opposite amplitude referenced to ground. An unbalanced signal is one where there is positive and ground. ALL signal amplitude is referenced either above or below ground, but never both. So as an example, a balanced 2V AC signal could be 1V positive, 1V negative. On a 2V unbalanced signal it would be 2V positive only. On both the signal is 2V AC, however how the signal is referenced to ground is different. This is a generalization, but is the basic concept. Balanced and unbalanced differ mainly in performance. In general a balanced signal is less prone to noise, and also exhibits total rejection of common-mode currents. This is due in theory to the equal and opposite poles which create cancellation of anything not part of the signal. Unbalanced signals tend to be more prone to noise and CMC. These differences by the way are the same regardless of frequency. The idea that somehow audio differs from RF is an incorrect assumption. It is when there is no proper consideration between balanced and unbalanced systems where things seem odd in ham radio. For instance, many hams use unbalanced tuners and open-wire feeders on balanced doublets, which creates problems in terms of actual balance (equal and opposite) on the feed-line regardless of whether or not an impedance match at the transmitter can be acheived; many times RF in shack is the result. Furthermore, the thinking that somehow impedance, SWR and resonance are somehow related to balanced/unbalanced systems is also incorrect. Those parameters are a matter of tuning, regardless of the system employed. However, how balanced/unbalanced systems REACT to SWR and impedance do differ, primarily due to the effects of CMC, reflection and impedance and how each system deals with each. As a general rule an antenna with a wide SWR curve is most often not always a good performer. It also does not say the resonance of the antenna is wide bandwidth. That's a misnomer. What it does say is the transmitter will pass maximum current over that range of frequencies, but what the total antenna system (tuner, feed-line, radiator) can do with that power is a different matter. Much of it may be lost, literally from losses outside of actual resonance. Think of it this way, a dummy load has a very wide bandwidth where an antenna with a high ERP (not created by directional gain) is usually very narrow. Again this is a huge over-simplification, but essentially it is factual ... a guideline. For instance, a resonant dipole which is brought closer to the ground (with all of the associated ground losses) has a broader curve as far as SWR. This is directly correlated to those losses, not because the resonance point of the antenna is broader. Also SWR is not an indicator of resonance or ERP. It's merely a measurement of forward & reflected power specifically at the point on the feed-line the measurement is taken. As far as impedance ... there is no such thing as a "DC impedance", impedance requires some frequency, meaning AC. Impedance is non-sequitur at DC. There is no "DC impedance" in an antenna, which is a tuned AC circuit. A choke balun does not create a ground on one side of a dipole. That is an incorrect assumption. A dipole is balanced with equal and opposite voltage poles, not positive and ground. Coaxial is not balanced. The shield is referenced to ground, and the center conductor has the full AC waveform above ground. The balun takes the balanced antenna and unbalanced coax, and coverts the signal to be appropriate ground reference for each. The impedance choking on the shield is simply an additional effect due to the nature of inductors and choking impedance, and unrelated to the bal-un conversion. Further more an UnUn is for feeding an unbalanced/unbalanced systems, like coax to a vertical or end-fed. Its purpose is solely to choke feedline CMC and transform impedance. Like 125ohm to 50ohm or some such. Do not confuse impedance matching, SWR and choking impedance with converting balanced to unbalanced. They are different parameters handled by a common device. For instance, on my balanced open-wire feeders I use two 340uh inductors (one on each pole) wired to earth ground. DC is shunted to ground on each leg, but with RF above about 1.2MHz the inductor is invisible, due to the high impedance presented. Also two capacitors are in series (one on each leg) which pass RF, but block DC. This effectively creates not only a bleed to ground for DC static but also blocks DC impulses. Of course, the devices are rated for the power levels involved. I use a balanced tuner and no baluns, since bal/unbal is not needed. Impedance matching is effected by the tuner alone, and the fully balanced system exhibits no feedline radiation since there was careful consideration of maintaining the balance (equal and opposite) of each pole of the system. Noise is very low on receive; due to a combination of very low-loss overall, CMC rejection, and very low-impedance (not just low-resistance) earth grounding. I care very little for feed-line SWR since it doesn't matter as long as I pass maximum current when transmitting when using the tuner, regardless of the SWR on the feed-line itself. A lot of the confusion with antennas is usually the act of propagation; in the case of antennas converting the energy from one state to another. In this case, electrons to photons. Loudspeakers are similar devices, they convert electrons into electromotive force which propagates as compression waves in air. Whether loudspeakers or antennas, impedance matching is critical in order to have current at maximum; regardless of whether the system devices are configured as balanced or unbalanced devices. John, W2WDX |
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